In recent years, a micro structure, which is a small in sized of hundreds micron meters, has been an object of public attention in the semiconductor manufacturing field. Such a micro structure is fabricated using a micro-machine technology, which is an application of semiconductor fine processing technology. A micro structure has been considered to be applied to a high-frequency device, including sensors and optical switches for optical communication. In general, a microstructure based on a micro-machine technology is fabricated using a semiconductor process, so that such a device can be integrated on a semiconductor chip together with a LSI for signal processing. Such a device is called “MEMS (Micro Electrical Mechanical System)” in the USA and “MIST (Micro System Technology)” in Europe.
An acceleration sensor can be fabricated using MEMS (MIST) technology. An acceleration sensor has been widely used for an airbag system of vehicle; a subsurface environment observation system for seismic activity; a seismic system for IT products; and so on. Japan Patent Publication No. H07-225240A describes a piezo-type of acceleration sensor using MEMS technology.    [Patent Related Publication 1] JP H07-225240A
An SOI wafer may be used for fabricating a semiconductor acceleration sensor based on MEMS technology. In general, a SOI wafer includes a balance oxide layer (SiO2); a supporting silicon layer (Si) formed on the balance oxide layer; a BOX oxide layer (SiO2) formed on the supporting silicon layer; and an active layer (Si) formed on the BOX oxide layer. When fabricating such a SOI wafer, a wafer state of a supporting substrate is shaped to have a predetermined thickness, and a surface of the wafer is mirror-finished. After that, oxide layers are formed on the both surface of the wafer as a BOX layer and a balance layer. Next, thus fabricated supporting substrate of a wafer state is laminated with a silicon wafer for an active layer. Next, the active layer is polished to have a predetermined thickness by a CMP process.
However, according to a SOI wafer fabricated by the above-described conventional method, an active layer is formed to have a uniform thickness and it is difficult to adjust or control the thickness of the active layer. Since the active layer is adjusted in thickness by processing its surface, the surface may be uneven, and therefore, a resist layer is hardly coated on the uneven surface. As a result, it is difficult to form a fine (micro) resist pattern on the wafer, and it is difficult to miniaturize a semiconductor device fabricated using such a wafer.
In a semiconductor acceleration sensor, fabricated using a SOI wafer, a mass is formed from a supporting substrate (Si) and beams are formed from an active layer (Si). A stopper layer may be formed from the active layer to restrict over movement (excess movement) of the mass. According to the conventional acceleration sensor, a sensitivity of piezo resistance elements, formed on the beams, is low; and protecting performance of the stopper layer is not sufficient.